Crash on "flow" queue state #100
Description
While load testing my environment I found out that whenever a queue is in "flow" state the plugin crashes with error message copied below (from RabbitMQ's crash.log file).
2020-07-08 18:39:39 =CRASH REPORT==== crasher: initial call: cowboy_stream_h:request_process/3 pid: <0.1663.0> registered_name: [] exception error: {{case_clause,flow},[{prometheus_rabbitmq_queues_collector,'-collect_mf/2-fun-3-',1,[{file,"src/collectors/prometheus_rabbitmq_queues_collector.erl"},{line,75}]},{prometheus_rabbitmq_queues_collector,'-collect_metrics/2-lc$^1/1-0-',3,[{file,"src/collectors/prometheus_rabbitmq_queues_collector.erl"},{line,102}]},{prometheus_model_helpers,create_mf,5,[{file,"/home/dead/Projects/rabbitmq/prometheus_rabbitmq_exporter/deps/prometheus/src/model/prometheus_model_helpers.erl"},{line,127}]},{prometheus_rabbitmq_queues_collector,mf,3,[{file,"src/collectors/prometheus_rabbitmq_queues_collector.erl"},{line,94}]},{prometheus_rabbitmq_queues_collector,'-collect_mf/2-lc$^4/1-2-',3,[{file,"src/collectors/prometheus_rabbitmq_queues_collector.erl"},{line,75}]},{prometheus_rabbitmq_queues_collector,collect_mf,2,[{file,"src/collectors/prometheus_rabbitmq_queues_collector.erl"},{line,75}]},{prometheus_collector,collect_mf,3,[{file,"/home/dead/Projects/rabbitmq/prometheus_rabbitmq_exporter/deps/prometheus/src/prometheus_collector.erl"},{line,153}]},{prometheus_registry,'-collect/2-lc$^0/1-0-',3,[{file,"/home/dead/Projects/rabbitmq/prometheus_rabbitmq_exporter/deps/prometheus/src/prometheus_registry.erl"},{line,86}]}]} ancestors: [<0.1662.0>,<0.632.0>,<0.631.0>,rabbit_web_dispatch_sup,<0.616.0>] message_queue_len: 0 messages: [] links: [#Port<0.329>,<0.1662.0>] dictionary: [{{xtype_to_module,topic},rabbit_exchange_type_topic},{{xtype_to_module,direct},rabbit_exchange_type_direct},{{xtype_to_module,headers},rabbit_exchange_type_headers},{{xtype_to_module,fanout},rabbit_exchange_type_fanout}] trap_exit: false status: running heap_size: 10958 stack_size: 27 reductions: 12436045